I listen to classical music, and it's a tough question because most music will tend to be quite varied. Most full length compositions alternate mood, tempo, etc., in the movements. The easiest solution is that most record labels do put out compilations of classical music to fit every mood. There are numerous collections like "the most relaxing classical music ever" and "classical music for lovers," etc. etc. etc. If you go to Amazon and type in "classical music and relax" or "classical music and romance" in the Music category you'll get a few pages worth of discs to choose from. Some are cheaper than others, but as long as you go with one of the major labels you're guaranteed good recordings by major artists. (EMI, Deutsche Grammophon, Decca, Philips, Sony, BMG/RCA.)
My tastes in classical music range quite a bit. Most of the stuff I listen to I wouldn't necessarily recommend to a newcomer! If you are still looking for more "relaxing" things a lot of the selections on the page I linked would be good. For some choral works the Vespers by Rachmaninov are quite amazing, performed without orchestral accompaniment. This recording by Shaw is well known:
Another composer to explore is an Estonian, Arvo P?rt. A lot of his best known works are based on the Russian liturgical style. This disc of his orchestral works contains a nice selection:
You may like this, it works for me when doing some work at home on the computer...Just click one of the mixes and you can sample the album on the right side first.. or at the bottom you can download the entire album for free!